Hi Navneet,
ViOP is fast becoming popular with upcoming call center as this is a cheaper way of using the internet to make/recieve calls. Any how for a 200 seater call center i do not think VoIP can solve the purpose, but any way u can use a dedicated web server that u would need to install and not a shared server, secondly there are several technology and hardware available to enhance voice quality and reduce packet losses. I would need to know if your call center is outbound or inbound or blended and how many call per seat do u expect in a shift and the duration of each call.
Regards
Amit Naagar